Improvement of superconducting properties in high-critical-temperature oxide superconducting composite tapes using multiscale approach

提出了一种新的高精度测量Bi 2223/Ag/Ag合金高临界温度氧化物超导体介观组分力学性能的方法。在此基础上，分析了新研制的超导带材在热压和机械载荷作用下的介观应力和应变状态。从复合带材设计优化的角度研究了变形与超导性的关系。（1）用去银带材直接测量纤维的弹性模量对完全去银的复合带材进行了拉伸试验。由于Ag的屈服强度和加工硬化系数很低，Bi 2223细丝的体积分数约为Ag的两倍，因此Bi 2223细丝在去除Ag合金的带材中几乎承担了全部载荷。然后，Bi的弹性模量 关于我们 2223细丝的应力-应变曲线简单地从去除Ag合金的带材计算，其中Ag组分应力超过屈服强度。换句话说，所提出的方法给出了Bi 2223细丝性质的几乎直接测量。所获得的细丝弹性模量，160至170 GPa，远高于常规的Bi 2223/Ag/Ag合金带材。常规带材的长丝弹性模量也与通过提取的Bi 2223长丝获得的弹性模量一致。受控超压过程显著增加了长丝模量。虽然临界电流增加，但丝模量的差异很小。（2）用Ag合金剥离带直接测量Bi 2223灯丝的断裂应变由于Ag的屈服强度和加工硬化系数很低，估计Bi 2223灯丝的热残余应变最大为0.004%。该值约为去除Ag合金的带材的断裂应变的5%。在评价灯丝断裂应变时，可以忽略灯丝的残余应变，直接由去除银合金带的断裂应变来计算灯丝的断裂应变。（3）复合材料中细丝断裂应变测量的准确性在77 K下同时测量了带材的力学性能和超导性能，结果表明，细丝断裂发生在带材最终断裂之前。考虑到这一事实和所有的热历史，复合带的应力-应变曲线进行了总结。在这里，银合金的应力-应变曲线估计由其成分的维氏硬度数据。实验结果与模拟结果吻合较好，表明银合金去除法计算的单丝断裂应变与复合材料中的单丝断裂应变吻合较好。（4）不同制备方法对Bi 2223/Ag/Ag合金带材的断裂应变的影响Bi 2223/Ag/Ag合金带材的断裂应变基本相同，约为0.08%。少

A new measurement method with higher accuracy was developed for mechanical properties of mesoscopic components in Bi2223/Ag/Ag alloy high-critical-temperature oxide superconductors. Then, mesoscopic stress and strain states under thermal and mechanical loading were analyzed for newly developed superconducting tapes with the controlled-overpressure sintering method. The correlation between the deformation and superconductivity were studied from the viewpoints of design optimization of composite tapes. The results are summarized as follows :(1)Direct measurement of elastic modulus of filament using Ag alloy removed tapeTensile tests were carried out for composite tapes from which Ag alloy component was completely removed. Bi2223 filaments carry almost the entire load in Ag alloy removed tapes because the yield strength and the coefficient of work hardening of Ag are quite low, and the volume fraction of Bi2223 filament is about twice as much as that of Ag. Then, the elastic modulus of Bi … More 2223 filament is simply calculated from the stress-strain curve of Ag alloy removed tapes where the Ag component stress is over the yield strength. In other words, this proposed method gives nearly direct measurement of Bi2223 filament properties. The obtained filament elastic moduli,160 to 170 GPa, are much higher than those of conventional Bi2223/Ag/Ag alloy tapes. The filament elastic modulus for conventional tapes also agreed with that obtained by extracted Bi2223 filaments. The controlled-overpressure process dramatically increases the filament modulus. The difference in filament moduli were minimal though the critical current increase.(2)Direct measurement of fracture strain of filament using Ag alloy removed tapeSince the yield strength and the coefficient of work hardening of Ag are quite low, the thermal residual strain of Bi2223 filament is estimated as 0.004% at most. This value is about 5% of the fracture strain of Ag alloy removed tapes. Then, the residual strain of filament can be ignored in the evaluation of the fracture strain of filament, and the fracture strain of filament is directly obtained by the fracture strain of Ag alloy removed tapes.(3)Accuracy of measured fracture strain of filament in compositesThe simultaneous measurement of mechanical and superconducting properties of tapes at 77K indicated that the filament fracture occurred slightly before the final tape fracture. Taking into account this fact and all of the thermal history, the stress-strain curves of composite tapes were summing up. Here, the stress-strain curves of Ag alloy were estimated by using Vickers hardness data of its component. The good agreement between experiment and simulation indicated that the filament fracture strain evaluated by the Ag alloy removed method agreed well with that in composite tapes.(4)Effect of processing method on fracture strain of filamentThe filament fracture strains, about 0.08%, are almost the same without respect to the fabrication method of Bi2223/Ag/Ag alloy tapes. Less